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This invention relates to improved methods and apparatus for completing wells in unconsolidated subterranean zones, and more particularly, to improved methods and apparatus for completing such wells whereby the migration of fines and sand with the fluids produced therefrom is prevented.
Oil and gas wells are often completed in unconsolidated formations containing loose and incompetent fines and sand which migrate with fluids produced by the wells. The presence of formation fines and sand in the produced fluids is disadvantageous and undesirable in that the particles abrade pumping and other producing equipment and reduce the fluid production capabilities of the producing zones in the wells.
Heretofore, unconsolidated subterranean zones have been stimulated by creating fractures in the zones and depositing particulate proppant material in the fractures to maintain them in open positions. In addition, the proppant has heretofore been consolidated within the fractures into hard permeable masses to reduce the migration of formation fines and sands through the fractures with produced fluids. Further, gravel packs which include sand screens and the like have commonly been installed in the wellbores penetrating unconsolidated zones. The gravel packs serve as filters and help to assure that fines and sand do not migrate with produced fluids into the wellbores.
In a typical gravel pack completion, a screen is placed in the wellbore and positioned within the unconsolidated subterranean zone which is to be completed. The screen is typically connected to a tool which includes a production packer and a cross-over, and the tool is in turn connected to a work or production string. A particulate material, which is usually graded sand, often referred to in the art as gravel, is pumped in a slurry down the work or production string and through the cross over whereby it flows into the annulus between the screen and the wellbore. The liquid forming the slurry leaks off into the subterranean zone and/or through the screen which is sized to prevent the sand in the slurry from flowing therethrough. As a result, the sand is deposited in the annulus around the screen whereby it forms a gravel pack. The size of the sand in the gravel pack is selected such that it prevents formation fines and sand from flowing into the wellbore with produced fluids.
A problem which is often encountered in forming gravel packs, particularly gravel packs in long and/or deviated unconsolidated producing intervals, is the formation of sand bridges in the annulus. That is, non-uniform sand packing of the annulus between the screen and the wellbore often occurs as a result of the loss of carrier liquid from the sand slurry into high permeability portions of the subterranean zone which in turn causes the formation of sand bridges in the annulus before all the sand has been placed. The sand bridges block further flow of the slurry through the annulus which leaves voids in the annulus. When the well is placed on production, the flow of produced fluids is concentrated through the voids in the gravel pack which soon causes the screen to be eroded and the migration of fines and sand with the produced fluids to result.
Incomplete packing of the interval may be caused by the liquid in the gravel slurry flowing into more permeable strata in the upper end of the formation interval and/or through the openings in the upper portion of the screen before sufficient gravel has been transported to the bottom of the completion interval.
In attempts to prevent the formation of sand bridges in gravel pack completions, special screens having internal shunt tubes have been developed and used. While such screens have achieved varying degrees of success in avoiding sand bridges, they, along with the gravel packing procedure, are very costly.
U.S. Pat. No. 4,945,991, which is incorporated herein by reference, discloses methods for gravel packing an interval of a wellbore wherein perforated shunts or conduits are provided on the external surface of the screen which are in fluid communication with the gravel slurry as it enters the annulus in the wellbore adjacent the screen. This method does not prevent the formation of such bridges where the liquid from the slurry is lost to the upper part of the gravel pack screen.
U.S. Pat. No. 5,934,376, which is incorporated herein by reference, discloses a method, basically comprising the steps of placing a slotted liner or perforated shroud with an internal sand screen disposed therein, in the zone to be completed, isolating the perforated shroud and the wellbore in the zone and injecting particulate material into the annuli between the sand screen and the perforated shroud and the wellbore to thereby form packs of particulate material therein. The system enables the fluid and sand to bypass any bridges that may form by providing multiple flowpaths via the perforated shroud/screen annulus and/or wellbore/screen annulus. See also Lafontaine, et al.: xe2x80x9cNew Concentric Annular Packing System Limits Bridging in Horizontal Gravel Packs,xe2x80x9d paper 56778 presented at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Tex., October 3-6, which is incorporated herein by reference.
U.S. Pat. No. 5,165,476, which is incorporated herein by reference, discloses a method and apparatus for gravel packing an interval of a wellbore wherein a permeable screen having a means for restricting fluid flow from the screen-wellbore annulus into the upper portions of the screen is positioned adjacent the wellbore interval. The flow-restrictive means may be comprised of a material which remains substantially solid during circulation of the gravel slurry but preferably can be removed, e.g., by melting or dissolving, after the gravel has been placed. However, this method does not provide multiple flow-paths, or prevent the problem of premature liquid loss from the gravel slurry to the upper end of the formation interval.
Thus, there are needs for improved methods and apparatus for completing wells in unconsolidated subterranean zones whereby the migration of formation fines and sand with produced fluids can be economically and permanently prevented while allowing the efficient production of hydrocarbons from the unconsolidated producing zone.
The present invention provides improved methods and apparatus for completing wells, and optionally simultaneously fracture stimulating the wells, in unconsolidated subterranean zones which meet the needs described above and overcome the deficiencies of the prior art.
The improved methods include the steps of placing a perforated shroud having an internal sand screen disposed therein whereby an annulus is formed between the sand screen and the perforated shroud in an unconsolidated subterranean zone, and injecting particulate material into the annulus between the sand screen and the perforated shroud and into the zone by way of the perforated shroud. Fluid flow from the shroud-screen annulus out through the upper portions of the perforated shroud is restricted during the gravel placement to prevent premature liquid loss to the upper end of the formation interval.
To improve the performance of the system in reducing the potential of screen-out or forming sand bridges inside the shroud-screen annulus, the number of holes or perforations on the shroud is decreased to an optimized number during the gravel packing operation. However, the number of holes on the shroud is preferably increased during the production phase to accommodate production flow without restriction.
A method of preparing perforations on a shroud is included wherein a number of perforations on the shroud is selected to be installed with screen or filter medium plate. The screen/filter plate can either be threaded or welded to the shroud so that it covers the perforations. The screen/filter is then coated or plated with a layer of dissolvable, meltable or erodable material to completely shut off the flow. After the placement of gravel in the wellbore, the material is removed from the screen/filter, allowing perforations to open up for more flow paths during production of the well.
Materials suitable for application in the improved methods include magnesium oxide/magnesium chloride/calcium carbonate mixtures, oil soluble resins, waxes, soluble polymers, etc. In one example, a paste form of a magnesium oxide/magnesium chloride/calcium carbonate mixture is put on the screen/filter plates, and allowed to cure before installation of the perforated shroud system down hole. After the gravel placement, a flush of weak HCl is applied into the wellbore and allowed to soak through the gravel pack. The coated material on the screen/filter plates is thereby removed.
Other suitable materials employ other mechanisms such as temperature, oil solubility, internal breaker or flow shear stress to remove them from the plates. Other methods such as using ceramic discs to cover the perforations and relying on explosive charges or sonic waves to rupture or break up the discs are also applicable.
During circulation of the gravel slurry, the flow of liquid from the slurry through the upper portions of the perforated shroud is restricted so that there is little, if any, premature liquid loss through the upper portions of the perforated shroud, thereby reducing the possibility of sand bridges being formed in the annulus. After the gravel has been deposited around the screen, fluid flow is re-established through substantially the full length of the perforated shroud.
The permeable pack of particulate material formed prevents the migration of formation fines and sand with fluids produced into the wellbore from the unconsolidated zone.
The unconsolidated formation can be fractured prior to or during the injection of the particulate material into the unconsolidated producing zone, and the particulate material can be deposited in the fractures as well as in the annuli between the sand screen and the slotted liner and between the slotted liner and the wellbore.
The apparatus of this invention include a perforated shroud having an internal sand screen disposed therein whereby an annulus is formed between the sand screen and the perforated shroud, a cross-over adapted to be connected to a production string attached to the perforated shroud and sand screen and a production packer attached to the cross-over. The perforated shroud has means for restricting fluid movement between the casing/shroud and shroud/screen annulus, including decreasing or increasing the number or size of holes or perforations on the shroud during gravel placement and during the production phase.
The improved methods and apparatus of this invention avoid the formation of sand bridges in the annulus between the slotted liner and the wellbore thereby producing a very effective sand screen for preventing the migration of fines and sand with produced fluids.
It is, therefore, a general object of the present invention to provide improved methods of completing wells in unconsolidated subterranean zones.